The field of this invention is instrumentation and in particular, instrument assemblies mounted in vessels, flowlines or the like that can be easily and safely removed as a unit.
A thermowell is an encapsulating container for a temperature-sensitive instrument and is well known in the art for placement in vessels, flowlines, reactors or other similar structures (referred to hereinafter collectively as "vessels") for indicating the temperature of the internal vessel environment. One type of thermowell may contain a thermocouple which provides an electrical signal indicative of the temperature within the vessel. This electrical signal is transmitted to known read-out equipment for providing the user with an actual temperature reading. Such thermowells are capable of providing extremely accurate readings of the internal temperature of the vessel. The need for extremely accurate temperature readings has increased as technology in certain processes has become more and more sophisticated. For example, U.S. Pat. No. 3,433,733 of Bunn, Jr. discloses a fluid catalytic cracking process wherein it is very important to the process to obtain accurate readings of the temperature of the mixture of oil vapor and catalyst which serves as the feed material for the reaction portion of the process. Such thermowells are generally configured as a rod-shaped device and may be sheathed with a hard facing for protection. But in many applications, such as the mounting of a thermowell in a vessel used in the fluid catalytic cracking process of the Bunn, Jr. patent, the life of the thermowell is too short due to erosion from the fluid flow within the vessel. One partial solution to the erosion problem has been to mount the thermowell within a fully enclosed sleeve that extends into the vessel. This solution has proved unsatisfactory at least in part because some of the fluid heat is dissipated by heat conduction through the vessel shell thereby causing the thermowell to provide less than accurate temperature-indicating signals. Another solution has been to expose the thermowell to the internal environment of the vessel but to provide some type of upstream shield attached to the vessel in order to divert direct fluid flow away from direct impingement against the thermowell. While these solutions have been helpful, certain problems remain. For example, shields mounted directly to the vessel have not been totally successful in preventing failure of the thermowells; and, users or operators in such processes such as disclosed in the Bunn, Jr. patent are wary of changing the thermowell during actual process operation since there is a possibility of fluid escape. Secondly, to replace the shields, the entire process must be shut down and the vessel evacuated so that a welder or other craftsman can enter the vessel and replace the shield. Further, the use of such shields mounted directly to the inside wall of the vessel is impractical in small vessels where it is difficult for a craftsman to get access to the interior of the vessel to install or replace the shield.